This invention relates to the modification of diisocyanate containing materials which results in increased binding capabilities. This invention also relates to the modification of diisocyanate containing materials which results in increased stability of the material. In another aspect, this invention relates to a method for producing a binder material especially suited for use in binding cellulosic materials into useful products such as boards and other construction materials. In a still further aspect, this invention relates to the conversion of cellulosic materials into useful products using a binder prepared by modifying diisocyanate containing materials with silica based compounds.
The manufacture of composition board products (sometimes referred to as engineered wood products) has become widespread and commonplace, primarily as a result of efforts to reduce consumption of exhaustible timber. As trees are harvested and millwork ensues, the residual bark, chips and sawdust are used to create board products for consumer applications. Such millwork reclaimed products serve many consumer needs while reducing consumption of exhaustible trees. These products are known as oriented strand boards, chipboards, particle boards and medium and high density fiberboards and are commonly used for (but not limited to) shelving, furniture, flooring, panelling, cabinets, doors, roofing, underlayment and sheathing construction materials.
Composition wood products are ordinarily formed using resins, ureas, phenols and formaldehyde. Often these products must be further processed with laminations and various structural and cosmetic treatments for consumer use and acceptance. While these manufactured products offer economic and conservation advantages and exhibit several characteristics of solid wood products, they are generally limited in application and are formed using chemical binders with undesirable and adverse environmental and health impacts. Furthermore, most technologies associated with formation of engineered wood products require reduction of wood to usable fiber form; employ wet chemistries; and require high temperatures and high pressures. Thus they require high energy consumption and raise environmental issues relating to disposal of process wastes and waste water reclamation and clarification.
Current U.S. consumption of adhesive resin solids to bond primary glued wood products exceeds 1.25 million metric tons. Approximately 50% of this volume comprises urea-formaldehyde (UF) resin; 45% comprises phenol-formaldehyde (PF) resins; with the balance distributed to synthetics. The industry has historically relied on UF and PF resins due to consideration of lower cost, experience, and performance. However, continuing environmental factors are now prompting new adhesive developments to provide safer, low emission products with high bonding performance. The synthetics, mainly polyvinyl acetates and polymeric MDI have gained acceptance but are still not cost competitive and contain some volatile monomers. It should be noted that MDI is preferred in industrial practices to TDI, HDI, NDI, and their derivatives, MDI being less volatile.
Methods to manufacture isocyanates have been known since the late 1800""s. Early investigations converted highly volatile cyanic acids to isocyanates. Cyanic acid is highly polymeric and hydrolyses in water to form ammonia and carbon dioxide. The salts and esters of its isomer are isocyanates. In these early methods, primarily the Curtius, Lossen, and Hoffman, commercial production was prohibitive due to instability hazards, reaction limitations of aliphatic isocyanates (since aromatic isocyanates react with water), and cost of process. These methods can be described as: 
The commercially viable method to produce isocyanates relies on phosgenation of a primary amine, developed by Hentschel in 1884. This method to produce isocyanates, both aromatic and aliphatic, can be described as: 
Isocyanates were initially used to produce herbicides and insecticides from their associated carbamates and substituted areas. As development progressed, addition of aromatic and aliphatic compounds resulted in diisocyanates, substances exhibiting polymeric properties conducive to elastomeric urethane products. These compounds and their derivatives, based on toluene, phenylmethane, naphthalene, and hexamethylene, (respectively TDI, MDI, NDI, and HDI) vary widely in properties, toxic hazards, and volatilities. Included in the range of products from these diisocyanates are thermoplastics, foams, fiber forming polymers, coatings, and adhesives. These derivatives in pure form yield products where maximum linear strength is desirable. However, some are subject to reduction of binding capability (e.g., adhesive bonding strength), wherein storage life and ambient temperatures cause decomposition of the diisocyanate. This reaction is commonly referred to as dimerization and can be depicted as: 
The loss of binding capability is dependent upon varying rates of reaction of the respective diisocyanates to polymerize.
The current practical method to avoid excessive dimerization has been to reduce the purity of a diisocyanate. As an example, MDI is produced from aniline (phenylamine) and formaldehyde, reacted together with hydrochloric acid as a catalyst. This condensation is then followed by phosgenation: 
Dependent upon the ratio of reactants and distillation of the existing MDI for purity, consecutive phosgenation of numerous polyamines produce a crude liquid containing diisocyanates, triisocyantes, and polyisocyanates. However, the resultant liquid isomer, although more stable than pure MDI, suffers loss of strength from the presence of these impurities.
This invention relates to the modification of TDI, MDI, HDI, NDI, and any derivatives thereof. More particularly, it relates to a method of incorporating derivatives of silicon into resin blends including but not limited to any variety of diisocyanates associated with prepolymeric or polymeric materials, polyols, and polyisocyanates to increase adhesive bond strength potential. Additionally, as the silicon content is increased, the modified resin is increasingly tolerant to dilution by water wherein adhesive bond strength potential is extended to produce suitable product for commercial applications at low cost.
The invention includes a method for preparing a binder material for cellulosic products which comprises providing a polymeric resin base material preferably comprising 4,4xe2x80x2 or 2,4xe2x80x2 methylene diisocyanate diphenylene in the range of from about 20% to about 55% by weight and a functionality greater than about 2, forming a colloidal gel from silica and a basic solution, and incorporating the colloidal gel into the polymeric base material at a temperature, rate of addition and agitation level to achieve a substantially homogeneous binder material. The preferred polymeric resin base material is PAPI or similar diisocyanate containing polymeric by-products obtained as a result of the manufacture of MDI. The colloidal gel is preferably formed by addition of an aqueous solution of an alkali metal hydroxide to solid silica. The amount of colloidal silica incorporated into the polymeric base resin is preferably in the range of from about 40% to about 60% of the molecular weight of the NCO content of the base polymer.
In another aspect, the invention relates to a method for increasing the binding capability and/or stability of diisocyanate containing polymeric resins, some of which have heretofore been considered by-products of no, or limited, utility in the bonding of cellulosic products. By modifying such compositions with silica based materials a new binder material is formed which displays enhanced binding capabilities and stability as compared to the unmodified base resin material. Such new binders can be further diluted with water and used to economically produce bonded cellulosic structures useful in a wide range of applications.
The invention also relates to preparing structural shaped articles, including boards, from cellulosic materials (for example, wood fibers) using wet or dry processes wherein the binder material is prepared as described above. The resulting products have excellent physical properties and can be produced at a substantially lower cost than similar products made using conventional binders.